Method and Apparatus for Producing Core-Shell Calcium Hydroxide-Calcium Carbonate Particles

ABSTRACT

The present disclosure describes a method for preparing calcium carbonate (CaCO3) coated calcium hydroxide (Ca(OH)2) particles. The method includes introducing liquid carbon dioxide into a reaction vessel, introducing calcium hydroxide particles into the reaction vessel, and effectively mixing the calcium hydroxide particles into the liquid carbon dioxide. The method further includes inducing a phase change in the liquid carbon dioxide so as to coat the calcium hydroxide in dry ice. In a different embodiment, liquid carbon dioxide may be introduced into a throttle valve inducing a phase change into a mixture of a gaseous carbon dioxide and a solid dry ice, and calcium hydroxide particles can be introduced into an exit stream with said mixture, inducing heterogeneous nucleation of the dry ice. In addition, both embodiments include sublimating the dry ice after a predetermined residence time to control the thickness of the calcium carbonate coating on the calcium hydroxide particles.

FIELD OF INVENTION

This present disclosure relates to the production of fillers, and morespecifically, the production of core-shell calcium hydroxide-calciumcarbonate particles.

BACKGROUND OF THE INVENTION

The use of fillers in polymer compositions, paints, and coatings is wellknown and established in literature. Fillers usually impart enhancedproperties to the final product, including mechanical, optical, physicalas well as fire retardancy properties. U.S. Pat. No. 9,493,658 and U.S.Pat. No. 6,310,129 provide suitable techniques for the use of fillers.Different commercial inorganic powdered fillers such as calciumcarbonates, talcs, clays, gypsum, barytes, feldspar and silicates arecurrently widely in use. Nevertheless, the application of ground mineralfillers is limited by their relatively large sizes, as indicated in U.S.Pat. No. 6,310,129. Thus, micro-size fillers are usually synthesizedchemically, which makes them much more costly.

Synthetic Ca(OH)₂ filler in polyvinyl chloride (PVC) neutralizes thetoxic chlorine gas produced in the event of PVC combustion. The fireretardancy of Ca(OH)₂, on the other hand, is questionable, since Ca(OH)₂reacts exothermically at relatively low temperature with CO₂ in presenceof air yielding CaCO₃, rather than decomposing endothermically to itsoxide upon heating in the presence of air as indicated in U.S. Pat. No.6,310,129. However, Ca(OH)₂ additive slowly reacts with atmospheric CO₂to yield CaCO₃, which may limit functionality of Ca(OH)₂. Nevertheless,commercial Ca(OH)₂ is still used as an additive for differentthermosetting resins to improve the tracking resistance ofelectrical/optical instruments, as indicated by U.S. Pat. No. 9,493,658,U.S. Pat. No. 6,310,129 and U.S. Pat. No. 7,883,681.

Carbide lime coproduced during acetylene manufacturing consists of 70-85% wt/wt Ca(OH)₂ and 5 - 25% wt/wt CaCO₃ in the form of shell onto theCa(OH)₂ grains as indicated in U.S. Pat. No. 7,883,681. Carbide lime hasbeen found an effective filler in many products owing to its multifoldproperties. Carbide lime is used for waste acid neutralization, gasscrubbing and desulphurization, pH control in sewage and water treatmentplants, production of building blocks and paving material,dehalogenation as well as the manufacturing of calcium magnesium acetateand calcium hypochlorite. These uses for carbide lime are indicated inU.S. Pat. No. 6,310,129 and U.S. Pat. No. 5,997,883 and F.A. Cardoso etal. “Carbide lime and industrial hydrated lime characterization”, PowderTechnol., 2009, doi: 10.1016/j.powtec.2009.05.017. Carbide lime is alsoan effective antibacterial, anti-viral, and anti- fungal agent asdescribed in U.S. Pat. No. 6,310,129 and U.S. Pat. 7,883,681. Carbidelime is ground and screened to collect particles of desired sizes for agiven application.

However, the use of carbide lime is limited by its greyish color due tothe coke used during the acetylene gas production. Thus, all resinmolded products utilizing the processed raw carbide lime have darkcolors as taught in U.S. Pat. No. 7,883,681. Therefore, syntheticcore-shell Ca(OH)₂ — CaCO₃ particles (also referred to herein as calciumcarbonate-coated calcium hydroxide particles) have been prepared.

One method of preparing the calcium carbonate-coated calcium hydroxideparticles is through blowing CO₂-containing gas, e.g., flue gas, into abed containing Ca(OH)₂ particles as described by Meade in U.S. Pat. No.7,883,681. In addition, exposure time helps controlling the thickness ofthe CaCO₃ coating, however, the process is poorly reproducible, mainlydue to particle collision. Collision deteriorates part of the coating,blocks particles from reacting, and contributes to major particleaggregation as shown in U.S. Pat. No. 7,883,681. To overcome thislimitation, U.S. Pat. No. 9,493,658 teaches the preparation of calciumcarbonate-coated calcium hydroxide particles upon reacting finely groundcommercial Ca(OH)₂ particles with dry ice. In one design, Ca(OH)₂particles and dry ice are added to a silo from two separate ports.Mixing between the reactants is enabled by gravity settling of theparticles in sublimating dry ice. Another design allows for a very briefmixing of the reactants in a Hobart mixer prior to introducing thereactants to the silo from a single port. This approach addressed thelimitations reported previously and was successful in producing a moreconsistent and even particles having 70 - 95 wt% Ca(OH)₂ and 5 - 30 wt%CaCO₃ surface coating as shown in U.S. Pat. No. 9,493,658.

However, it is noted that gravity settling within the silo may not beideal to control the thickness of the CaCO₃ coating. While residencetime depends on the height of the silo and Ca(OH)₂ particle size asindicated in H. Scott Fogler, “Elements of chemical reactionengineering” Chem. Eng. Sci., 1987, doi: 10.1016/0009-2509(87)80130-.6,less control over the calcium carbonate-coated calcium hydroxideparticle size distribution is achieved within a silo. Moreover, thecalcium carbonate-coated calcium hydroxide particle size is mainlycontrolled through selecting the size of the Ca(OH)₂ reactant particles.For example, reacting ~ 44 µm Ca(OH)₂ particles with dry ice having amesh size of minus 12 - plus 18 in the form of flakes produces calciumcarbonate-coated calcium hydroxide particles of 0.1 - 75 µm. For a givensilo, the mass ratio of the reactants is used to control the calciumcarbonate-coated calcium hydroxide product specifications as indicatedin U.S. Pat. No. 9,493,658. Lastly, mixing of the solid reactants priorto introducing the reactants to the silo may lead to Ca(OH)₂ particleaggregation, especially given the particle small size and thecorresponding surface energy, as taught in M. Husein, “Preparation ofnanoscale organosols and hydrosols via the phase transfer route”,Journal of Nanoparticle Research. 2017, doi: 10.1007/s11051-017-4095-0.

Modeled after carbide lime, the calcium carbonate-coated calciumhydroxide particles produced from reacting Ca(OH)₂ with dry ice alsohave proven antibacterial, antifungal, and antiviral attributes as wellas significant pH adjustment property as indicated in U.S. Pat. No.9,493,658. These attributes make these synthetic calciumcarbonate-coated calcium hydroxide particles an ideal filler fordifferent commercial products. Preliminary testing in the SouthwestResearch Institute, “Final Report of Southwest Research Institute (SwRI)Project 20637 (Proposal No. 01-72445) “Mold Resistance Efficacy Testingof Paint with ZeroMold Additive,” 2015 and Southwest Research Institute,“Laboratory Testing Results,” 2014 showed that resin-molded products andpaints mixed with calcium carbonate-coated calcium hydroxide particlesimpart significant sterilizing properties, including bactericidal,fungicidal, and virucidal attributes. These antimicrobial attributes areinduced by the high Ca(OH)₂ content and is expected to last for up to ahundred years, per the accelerated aging testing as is indicated in U.S.Pat. No. 9,493,658.

In addition, in Vance et al. (2015) ‘Direct Carbonation of Ca(OH)(₂)Using Liquid and Supercritical CO₂: Implications for Carbon-NeutralCementation’, Industrial & engineering chemistry research, 54(36), pp.8908-8918. doi:10.1021/acs.iecr.5b02356, the carbonation of Ca(OH)₂ uponplacing in liquid CO₂ was investigated. An isothermal process to ventout the liquid CO₂ was used. Analysis of the reaction kinetics showedthat Ca(OH)₂ reaction in liquid CO₂ is rapid (~ 80% conversion in 2 h).This suggests that the product CaCO₃ layer is non-passivating. Thepressure and temperature had little effect on the carbonation rate.Furthermore, scanning electron microscope (SEM) images for thecarbonated Ca(OH)₂ have indicated the formation of calcite layers on thesurfaces of Ca(OH)₂ grains. Irregular growth, nonuniform morphologicalstructure, and exfoliation of the initially formed CaCO₃ surface layers(terracing effect) are the main reasons behind the non-passivatingcalcite layer formed on top of Ca(OH)₂. The materials produced using theprocedure of Vance et al. were tested for biocidal activity and theresults showed low effective biocidal activity. This was due to the highextent of particle agglomeration as well as inconsistent CaCO₃ film. Thedrawbacks in morphology contributed to less effective biocidalparticles.

Furthermore, according to Dheilly, R.M, J Tudo, Y Sebaïbi, and MQueneudec. “Influence of Storage Conditions on the Carbonation ofPowdered Ca(OH)₂ .” Construction & building materials 16, no. 3 (2002):155-161, a drawback of the reaction of Ca(OH)₂ with gaseous CO₂ is thatit occurs slowly at the temperatures associated with the throttlingprocess, especially in absence of moisture.

SUMMARY OF THE INVENTION

According to various aspects to the present invention, there is provideda method for preparing calcium carbonate (CaCO₃)-coated calciumhydroxide (Ca(OH)₂) particles. The method includes introducing liquidcarbon dioxide into a reaction vessel, introducing calcium hydroxideparticles into the reaction vessel, and effectively mixing the calciumhydroxide particles into the liquid carbon dioxide. The method furtherincludes inducing a phase change in the liquid carbon dioxide so as tocoat the calcium hydroxide in dry ice. In addition, the method includessublimating the dry ice after a predetermined residence time to controlthe thickness of the calcium carbonate coating on the calcium hydroxideparticles.

The method may include the liquid carbon dioxide being introduced intothe reaction vessel at a pressure of 8 MPa and a temperature of —25° C.

Alternatively, the method may include the liquid carbon dioxide beingintroduced into the reaction vessel at a pressure range of 0.518 MPa to16 MPa and a temperature range of -56.56° C. to 30.98° C.

The introduction of calcium hydroxide particles into the reaction vesselmay include feeding the calcium hydroxide particles into an auxiliarychamber, flushing the calcium hydroxide particles in the auxiliarychamber with the liquid carbon dioxide and introducing the mixture intothe reaction vessel to be further mixed with the already present liquidcarbon dioxide.

Alternatively, the calcium hydroxide particles may be introduced intothe reaction vessel prior to the liquid carbon dioxide being introducedinto the reaction vessel.

The method may include a high-pressure reactor as the reaction vessel,the high-pressure reactor including a stirrer for mixing.

Alternatively, the method may include an inline mixer as the reactionvessel.

Inducing the phase change in the liquid carbon dioxide may be performedusing a throttle valve to flash the liquid carbon dioxide into dry ice.

The throttle valve may flash at a pressure of 0.1 MPa to create dry ice.

Alternatively, the throttle valve may flash at a pressure range of 0.01MPa to 0.518 MPa and a temperature lower than —56.56° C.

Controlling the thickness of the calcium carbonate coating on thecalcium hydroxide particles occurs over the predetermined residence timein a separator vessel at a pressure of less than or equal to 0.518 MPa.

The method may further include collecting gaseous carbon dioxide fromthe sublimation of the dry ice and inducing a phase change in thegaseous carbon dioxide to provide liquid carbon dioxide to be introducedinto the reaction vessel.

According to various aspects to the present invention, there is provideda system for producing calcium carbonate (CaCO₃)-coated calciumhydroxide (Ca(OH)₂) particles. The system includes a reaction vessel forreceiving liquid carbon dioxide and calcium hydroxide particles. Thesystem further includes a stirrer to effectively mix the liquid carbondioxide and calcium hydroxide particles, and a throttle valve forinducing a phase change to liquid carbon dioxide to coat the calciumhydroxide particles in dry ice. In addition, the system includes aseparator vessel for sublimating the dry ice after a predeterminedresidence time to control the thickness of the calcium carbonate coatingon the calcium hydroxide particles.

The system may include the liquid carbon dioxide being received by thereaction vessel at a pressure of 8 MPa and a temperature of —25° C.

Alternatively, the system may include the liquid carbon dioxide beingreceived by the reaction vessel at a pressure range of 0.518 MPa to 16MPa and a temperature range of -56.56° C. to 30.98° C.

The calcium hydroxide particles may be received by the reaction vesselthrough flushing the calcium hydroxide particles in an auxiliary chamberwith the liquid carbon dioxide and introducing the mixture into thereaction vessel to be mixed with the already present liquid carbondioxide.

Alternatively, the calcium hydroxide particles may be received by thereaction vessel prior to the liquid carbon dioxide being received by thereaction vessel.

Alternatively, the liquid carbon dioxide is received by the reactionvessel prior to the calcium hydroxide particles being received by thereaction vessel.

The system may include a high-pressure reactor as the reaction vesseland the high-pressure reactor including a stirrer for mixing.

Alternatively, the system may include an inline mixer as the reactionvessel.

The throttle valve of the system may induce a phase change by flashingthe liquid carbon dioxide at a pressure range of 0.01 MPa to 0.518 MPaand a temperature lower than -56.56° C.

Alternatively, the throttle valve of the system may induce the phasechange by flashing the liquid carbon dioxide to a pressure of 0.1 MPa.

Controlling the thickness of the calcium carbonate coating on thecalcium hydroxide particles occurs over the predetermined residence timein a separator vessel at a pressure of less than or equal to 0.518 MPa.

The system may further include a gaseous carbon dioxide outlet connectedto the separator vessel, where the gaseous carbon dioxide outletcollects gaseous carbon dioxide from the sublimation of the dry ice inthe separator vessel. The system may also include a return line with anin-line pressurization system connecting the gaseous carbon dioxideoutlet and the reaction vessel, where the return line with the in-linepressurization system may be configured to induce a phase change to thegaseous carbon dioxide to provide liquid carbon dioxide to be introducedinto the reaction vessel.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The embodiments of the present invention shall be more clearlyunderstood with reference to the following detailed description of theembodiments of the invention taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 depicts a system for producing calcium carbonate-coated calciumhydroxide entrapped into dry ice in accordance with an embodiment of theinvention;

FIG. 2 depicts a method of producing calcium carbonate-coated calciumhydroxide particles in accordance with a first production line of theexample system in FIG. 1 ;

FIG. 3 depicts a method of producing calcium carbonate-coated calciumhydroxide particles in accordance with a second production line of theexample system in FIG. 1 ;

FIG. 4 depicts a system for producing calcium carbonate-coated calciumhydroxide in accordance with another embodiment of the invention, wherethe end product gaseous CO₂ is recirculated into the system for furtheruse as liquid CO₂

FIG. 5 depicts a method of producing calcium carbonate-coated calciumhydroxide particles in accordance with a first production line of thesystem shown in FIG. 4 ;

FIG. 6 depicts a phase diagram of carbon dioxide (CO₂) showing thestability fields of the solid, liquid and vapor states;

FIG. 7 depicts a phase diagram of carbon dioxide (CO₂) showing theregions of thermodynamically stable state(s) of CO₂ (i.e. solid, liquidand vapor states) at different values of pressure (psia) and specificenthalpy H (Btu/lb_(m)), where pressure is provided along the Y-axis onthe left side of the phase diagram, specific volume ν (ft³/lb_(m)) isprovided along the Y-axis on the right side of the phase diagram andspecific enthalpy is provided along the X-axis of the phase diagram andother thermodynamic properties corresponding to given values of pressureand specific enthalpy, specific entropy S (Btu/(1b_(m))(R)), temperature(°F) and χ, weight fraction vapor, are also shown. The reference stateis saturated liquid CO₂ at -40° F., where specific enthalpy H=0 andspecific entropy S=0;

FIG. 8 depicts a system for producing calcium carbonate-coated calciumhydroxide particles in accordance with yet another embodiment of theinvention, where the calcium hydroxide particles induce heterogeneousnucleation of dry ice in an exit stream of the throttling valve; and

FIG. 9 depicts a method of producing calcium carbonate-coated calciumhydroxide particles in accordance with the example system in FIG. 8 .

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

The description, which follows, and the embodiments described thereinare provided by way of illustration of an example, or examples ofparticular embodiments of principles and aspects of the presentinvention. These examples are provided for the purposes of explanationand not of limitation, of those principles of the invention. In thedescription that follows, like parts are marked throughout thespecification and the drawings with the same respective referencenumerals.

By way of general overview, there is provided a method for preparingcalcium carbonate-coated calcium hydroxide particles in accordance witha preferred embodiment of the invention. The method generally involvesdispersing calcium hydroxide (Ca(OH)₂) particles in liquid carbondioxide (CO₂) and then flashing/throttling the particles to induce aphase change in the liquid carbon dioxide so it becomes dry ice. Theresultant dry ice entraps the Ca(OH)₂ particles within its solidstructure achieving enhanced coating of the particles with dry ice.

In contrast to existing methods, method 200 is advantageous in that itallows improved carbonation and enhanced control over the level ofcarbonation of the particles. This is achieved through solid-liquidmixing, which tends to achieve a more thorough mixing than solid-solidmixing. Major carbonation reaction, however, proceeds between the dryice and the entrapped Ca(OH)₂ particles, thereby allowing for moreuniform coating of the particles. In this method, carbonation proceedsat the same rate in all directions, including the radial direction. Asdiscussed below, the thickness of the CaCO₃ shell can be controlled byselectively reducing or increasing the residence time during which thecarbon dioxide remains as a dry ice coating on the calcium hydroxidecore, prior to sublimating the dry ice. This, in turn, permits bettercustomization of the structural properties of the resultant calciumcarbonate-coated calcium hydroxide particles such that the particles canbe used in a wider range of products or have a wider range ofapplications, such as use in fillers for plastics, papers, cement anddrywall. In addition, the resultant calcium carbonate-coated calciumhydroxide particles also have a higher biocidal activity, leading to anincreased number of uses, especially in environments where a biocidaleffect is advantageous. Method 200 further differs from existingproduction methods in that phase changes are induced in the carbondioxide from liquid phase to solid and gas phases, as opposed toexisting methods where the carbon dioxide changes from the solid phaseto the gas phase. In addition, method 200 is advantageous over existingproduction methods in that it has a significantly faster productiontime.

FIG. 1 depicts a system 100 for producing calcium carbonate-coatedcalcium hydroxide particles having two production lines 160 and 160A.The first production line 160 is depicted above the dotted line insystem 100 and the second production line 160A is depicted below thedotted line in system 100. The first production line 160 and the secondproduction line 160A may run in parallel and share certain componentsthat will be discussed further below.

The first production line 160 includes an insulated pressurized liquidCO₂ storage tank 104 (hereinafter referred to as storage tank 104)connected to an insulated high pressure reactor 124 through a CO₂ feedline 108, allowing liquid CO₂ to be sent from storage tank 104 to highpressure reactor 124. Gate valve 112 and in-line pressurization system116 may be positioned along CO₂ feed line 108. In addition to receivingliquid CO₂ from CO₂ feed line, high pressure reactor 124 also receivesCa(OH)₂ particles from Ca(OH)₂ feed line. High pressure reactor 124includes stirrer 128 for mixing the liquid CO₂ and the Ca(OH)₂particles. High pressure reactor 124 is connected to separator vessel136 via throttle valve 132, where throttle valve 132 flashes the liquidCO₂ surrounding the Ca(OH)₂ particles and separator vessel 136 receivesthe resultant dry ice. The dry ice then sublimates in separator vessel136 where the produced gaseous CO₂ is discharged through the connectedgaseous CO₂ outlet 140 and the produced calcium carbonate coatedhydroxide particles are discharged via the connected calcium carbonatecoated calcium hydroxide product particle outlet 144.

Insulated pressurized liquid CO₂ storage tank 104 stores liquid CO₂, andis readily available through commercial means. Storage tank 104 may beof any size, and in this current embodiment may be the standardized 50tonne storage tank that is typically supplied by tanker trucks.Typically, the storage tank 104 installation and associated piping andcontrols is part of vendor’s supply and service agreement.

Liquid CO₂ feed line 108 leads from storage tank 104 to high pressurereactor 124, allowing liquid CO₂ to be sent from storage tank 104 tohigh pressure reactor 124 (also known as a reaction vessel), where it isreceived by high pressure reactor 124 through a leak-proof port. In thecurrent embodiment, CO₂ feed line 108 includes going through gate valve112, and in-line pressurization system 116. Gate valve 112 allowscontrol of the flow of liquid CO₂ from storage tank 104 to high pressurereactor 124, and further allows the flow of liquid CO₂ to be safely shutoff. In the current embodiment, gate valve 112 is used, however inalternate embodiments, a ball valve may be used for the same function. Aperson skilled in the art will recognize that different valve types maybe used to control the flow of liquid CO₂ and for the safe shut off ofthe flow of liquid CO₂. In-line pressurization system 116 maintains thepressure within CO₂ feed line 108 to ensure that the CO₂ remains in aliquid state. While not necessary in the current embodiment, as theliquid CO₂ is already in the liquid state to be introduced intohigh-pressure reactor 124, in-line pressurization system 116 may alsochange the pressure, while maintaining lower temperature than thecritical temperature, if storage tank 104 holds CO₂ in a differentstate, such as gaseous CO₂, inducing a phase change from gaseous CO₂ toliquid CO₂. It will occur to a person skilled in the art that gate valve112 and in-line pressurization system 116 are optional, and that storagetank 104 may send liquid CO₂ to high-pressure reactor 124 without theneed for gate valve 112 or in-line pressurization system 116. In otherembodiments, first production line 160 may include gate valve 112 andlack an in-line pressurization system 116 or alternatively, productionline 160 may include in-line pressurization system 116 and lack gatevalve 112.

Ca(OH)₂ feed line 120 allows Ca(OH)₂ particles to be dispersed intohigh-pressure reactor 124. High-pressure reactor 124 includes stirrer128, allowing the Ca(OH)₂ particles to be further mixed with the liquidCO₂ to ensure that the Ca(OH)₂ particles are thoroughly distributed andcoated with the liquid CO₂.

The resulting mixture can then be sent through throttle valve 132 intoseparator vessel 136. The resulting mixture goes through a phase changewhile it passes through throttle valve 132, resulting in dry iceparticles containing calcium hydroxide particles.

The second production line 160A is similar in layout to the firstproduction line 160 but uses an inline mixer 148 in place a ofhigh-pressure reactor 124. The second production line 160A includesstorage tank 104 connected to inline mixer 148 via a CO₂ feed line 108A.Gate valve 112A and in-line pressurization system 116A are positionedalong CO₂ feed line 108A. Inline mixer 148 receives liquid CO₂ from CO₂feed line 108A, and further receives Ca(OH)₂ from Ca(OH)₂ feed line120A. The liquid CO₂ and Ca(OH)₂ are mixed in inline mixer 148 and sentthrough throttle valve 132A. Throttle valve 132A flashes the liquid CO₂surrounding the Ca(OH)₂ particles into dry ice, where it is received byseparator vessel 136. The dry ice sublimates, and the resulting productsof gaseous CO₂ and calcium carbonate coated calcium hydroxide particlesare discharged through their respective outlets, gaseous CO₂ outlet 140,and calcium carbonate coated calcium hydroxide product particle outlet144.

CO₂ feed line 108A leads from storage tank 104 to inline mixer 148,allowing liquid CO₂ to be sent from storage tank 104 to inline mixer148. CO₂ feed line 108A includes going through gate valve 112A, andin-line pressurization system 116A. Similar to gate valve 112 of firstproduction line 160, gate valve 112A of second production line 160Aallows control of the flow of liquid CO₂ from the storage tank 104, andfurther allows the flow of liquid CO₂ to be safely shut off. Similar toin-line pressurization system 116 of first production line 160, in-linepressurization system 116A of second production line 160A maintains thepressure within CO₂ feed line 108A to ensure the CO₂ remains in a liquidstate. Similar to first production line 160, gate valve 112A and in-linepressurization system 116A are optional. Liquid CO₂ is received intoinline mixer 148 through a leak-proof port.

Ca(OH)₂ particles may be dispersed into inline mixer 148 through Ca(OH)₂feed line 120A, where the Ca(OH)₂ particles may be further mixed withliquid CO₂ to ensure that the Ca(OH)₂ particles are thoroughly dispersedinto liquid CO₂. Inline mixer 148 may also have a stirrer to furtherpromote the dispersion of the Ca(OH)₂ particles. The resulting mixturecan then be sent through throttle valve 132A where dry ice is formed.The dry ice is then fed into separator vessel 136.

Separator vessel 136 allows the dry ice to settle and allows gaseous CO₂to leave without carrying particles with it. In both the firstproduction line 160 and second production line 160A, separator vessel136 may include a filter to aid in separating gaseous CO₂ and calciumcarbonate coated calcium hydroxide product particles. The filter (alsoknown as a mist eliminator) captures dust or particles that are leavingwith the CO₂ vapors. The mist eliminator is also generally used inevaporators and is known to persons skilled in the art. Furthermore,separator vessel 136 may include a pressure control module to change andmaintain pressure within separator vessel 136, and a heating element toaccelerate dry ice sublimation if needed.

Once the dry ice has been sublimated, the resultant products of gaseousCO₂ and calcium carbonate-coated calcium hydroxide products may becollected through gaseous CO₂ outlet 140 and calcium carbonate-coatedcalcium hydroxide product particle outlet 144 respectively.

In alternate embodiments, separator vessel 136 may be substituted withcyclone 832, where centrifugal forces and cyclonic separation allow theseparation of the calcium carbonate coated calcium hydroxide productparticles and the gaseous CO₂. Cyclone 832 will be further discussedbelow.

In another embodiment, system 100C depicts the same two production lines160 and 160A, however, gaseous CO₂ from gaseous CO₂ outlet 140 isreturned to the storage tank 104 via return line 156, after goingthrough in-line pressurization system 152. In-line pressurization system152 raises the pressure within return line 156 to induce a phase changein the CO₂, converting the gaseous CO₂ into liquid CO₂. It will occur toa person skilled in the art that in-line pressurization system 152 maybe any component that induces a phase change in the CO₂.

Returning to FIG. 1 , while system 100 depicts the two production lines160 and 160A for producing calcium carbonate-coated calcium hydroxide,it will occur to a person skilled in the art that the two productionlines 160 and 160A are not limited to running in parallel and may infact run independently.

FIG. 2 depicts method 200 for producing calcium carbonate coated calciumhydroxide particles using first production line 160.

In the first production line 160, block 205 depicts introducing liquidCO₂ into high-pressure reactor 124 from storage tank 104 via CO₂ feedline 108. Block 210 depicts feeding Ca(OH)₂ particles into high-pressurereactor 124 via Ca(OH)₂ feed line 120.

In the current embodiment of method 200, liquid CO₂ is received first byhigh-pressure reactor 124, and then Ca(OH)₂ particles are then dispersedinto high-pressure reactor 124. In other embodiments, it is contemplatedthat Ca(OH)₂ particles could be fed into the high-pressure reactorbefore the liquid CO₂. In another embodiment, it is contemplated thatCa(OH)₂ particles are placed into a small chamber or an auxiliarychamber, and then flushed with a small amount of liquid CO₂, prior tobeing moved into high-pressure reactor 124 to be further mixed withadditional liquid CO₂. The Ca(OH)₂ particles can be flushed with a smallamount of liquid CO₂ into a nozzle which directs the mixture into anopposing nozzle ejecting liquid CO₂. The spray from either nozzle isdesigned to overlap with one another, to further promote dispersion.

Once both liquid CO₂ and Ca(OH)₂ particles are received, they may bemixed/dispersed in high-pressure reactor 124 using stirrer 128. This isdepicted in block 215. Throughout the steps in blocks 205 to 215, theliquid CO₂(1) may be kept at a range of 0.518 MPa to 16 MPa and -56.56°C. to 30.98° C. In a preferred embodiment, the liquid CO₂ is kept at 8MPa and - 25° C., as this is easily achieved, and is readily used inindustry. The dispersion of the Ca(OH)₂ particles in liquid CO₂ is toensure a thorough and uniform coating of liquid CO₂ surrounding theCa(OH)₂ particles.

At block 220, the liquid CO₂ and Ca(OH)₂ particle mixture is then sentthrough a throttle valve 132. The liquid CO₂ undergoes a phase changefrom liquid into a solid, creating a dry ice shell surrounding theCa(OH)₂ particles.

In the current embodiment, the liquid CO₂ and Ca(OH)₂ mixture is sentthrough throttle valve 132, and the liquid CO₂ is flashed into dry iceat preferred normal atmospheric pressure of 0.1 MPa creating a dry iceshell surrounding the Ca(OH)₂ particle. Alternatively, the dry ice shellmay also be formed through throttle valve 132 at different temperaturesand pressures by undergoing a phase change. FIG. 6 is apressure-temperature phase diagram that depicts other conditions atwhich liquid CO₂ and dry ice can be obtained. FIG. 7 is apressure-enthalpy phase diagram depicting conditions at which dry icecan be formed from liquid CO₂ through an isenthalpic process. FIG. 6 canbe found at “CO₂ as a Refrigerant - Properties of R744” by AndrePatenaude published on May 14, 2015, located at the following URL:“https://emersonclimateconversations.com/2015/05/14/co2-as-a-refrigerant-properties-of-r744/”.The data for FIG. 7 can be found at Plank, R., and Kuprianoff, J., Z.ges. Kalte-Ind., 1, 1 (1929); Z. tech. Physik, 10, 99 (1929). It willoccur to a person skilled in the art that a change of state may beperformed through a change in pressure or temperature, and as such, theliquid CO₂ and Ca(OH)₂ mixture is not limited to being sent throughthrottle valve 132. Other apparatus or devices may be contemplated toaid in the change of state from the liquid CO₂ and Ca(OH)₂ mixture intodry ice.

The dry ice shell surrounding the Ca(OH)₂ particles are then fed intoseparator vessel 136. Pressure is maintained within separator vessel136, allowing the Ca(OH)₂ particles to react with the dry ice shell,affecting the thickness of the CaCO₃ shell. The longer pressure ismaintained, the thicker the CaCO₃ shell. The residence time of theCa(OH)₂ particles and the dry ice within separator vessel 136 whilepressure and temperature are maintained, correlates directly to thethickness of the CaCO₃ shell surrounding the Ca(OH)₂ particles. In apreferred embodiment, separator vessel 136 operates at 0.1 MPa, howeverseparator vessel 136 may be maintained at a pressure range between 0.01MPa to 0.518 MPa to thicken the CaCO₃ shell. In a preferred embodiment,separator vessel 136 may be maintained above —78.5° C. Once the desiredthickness of the CaCO₃ shell is achieved, the dry ice may be sublimated,producing gaseous CO₂. The dry ice may also be heated using a heatingelement, to further accelerate the sublimation process. This is depictedat block 225.

At block 230, the resulting calcium carbonate-coated calcium hydroxideproduct is collected. It will occur to the person skilled in the artthat the size of the calcium carbonate-coated calcium hydroxide productmay be controlled by the choice of the Ca(OH)₂ particle size that is fedinto high-pressure reactor 124 through Ca(OH)₂ feed line 120.

FIG. 3 depicts method 200A for producing calcium carbonate-coatedcalcium hydroxide particles using second production line 160A. Aspreviously mentioned, second production line 160A uses inline mixer 148instead of high-pressure reactor 124. Liquid CO₂ is introduced intoinline mixer 148 at block 205A and Ca(OH)₂ particles are fed into inlinemixer 148 at block 210A. The liquid CO₂ and Ca(OH)₂ is thenmixed/dispersed in inline mixer 148 at block 215A, before continuingthrough method 200. It will occur to the person skilled in the art thatmethod 200 may be used with different embodiments of systems used toproduce calcium carbonate-coated calcium hydroxide products.

FIG. 5 depicts method 200C for producing calcium carbonate coatedcalcium hydroxide particles using first production line 160 and furthertaking the gaseous CO₂ product and cycling it back to be reused in firstproduction line 160. Method 200C has similar steps to method 200, andblocks 205 to 230 follow the same process. Block 235 shows thecollection of gaseous CO₂ from gaseous CO₂ port 140. Block 240 depictsin-line pressurization system 152 inducing a phase change on the gaseousCO₂, changing it to liquid CO₂. It will occur to a person skilled in theart that similar to block 220, the phase change may not be limited to beinduced by a change in the pressure, but may also be a change intemperature, or a change in both temperature and pressure. FIG. 6depicts the temperatures and pressures that liquid CO₂ may be obtainedat.

Returning to FIG. 5 , once liquid CO₂ has been obtained, it is thenreintroduced into first production line 160 at storage tank 104, and isdepicted by the line between block 240 and block 205. It will occur to aperson skilled in the art that the recirculation of CO₂ may also beapplied to method 200A in second production line 160A.

The size of the resultant calcium carbonate-coated calcium hydroxideproduct particle collected may be determined based on the Ca(OH)₂particles fed into either high-pressure reactor 124 or inline mixer 148.The larger the Ca(OH)₂ particles fed into the system, the larger theresultant calcium carbonate coated calcium hydroxide product particles.Likewise, nanoparticle calcium carbonate-coated calcium hydroxideparticles can be achieved by feeding nanosized Ca(OH)₂ reactantparticles into the system.

A person skilled in the art will recognize that method 200 and method200A may be performed with particles other than Ca(OH)₂ particles.Particles may be fed into high-pressure reactor 124 through a feed linein method 200, where the particles are mixed with liquid CO₂.Alternatively, particles may be fed into inline mixer 148 through a feedline in method 200A, where the particles are mixed with liquid CO₂.

In other embodiments, different methods may be used to coat calciumhydroxide particles with calcium carbonate. For example, in analternative embodiment, liquid CO₂ may be flashed within a reactor bysuddenly dropping the pressure within the reactor to below 0.518 MPa.Dry ice forms on Ca(OH)₂ particles that were previously fed into thereactor, where the Ca(OH)₂ particles act as heterogeneous nucleationsites for the formation of said dry ice. The dry ice around the Ca(OH)₂particles reacts with the outer shell of the Ca(OH)₂ particles producinga shell of CaCO₃.

In a preferred embodiment, liquid CO₂ may be throttled to induce a phasechange into an exit stream wherein it is mixed with Ca(OH)₂ particlesand where, through heterogeneous nucleation, dry ice covered Ca(OH)₂particles are created. These particles are then collected in a cyclone,where the remaining dry ice and the calcium carbonate (CaCO₃)-coatedcalcium hydroxide Ca(OH)₂ particles are separated and collected. Anadvantage of this embodiment is that throttling to induce a phase changeto the liquid CO₂ is simple to implement leading to a system with lowmaintenance and less failure points. Another advantage of thisembodiment, is that similar to method 200, the production time of thisembodiment is significantly faster than that of existing productionmethods. In addition, similar to method 200, the resultant calciumcarbonate-coated calcium hydroxide particles from this embodiment alsohave a higher biocidal activity in comparison to those in the previouslycited Vance et al. (2015) ‘Direct Carbonation of Ca(OH)(₂) Using Liquidand Supercritical CO₂: Implications for Carbon-Neutral Cementation’,leading to an increased number of uses, especially in environments wherea biocidal effect is advantageous.

An exemplary system for implementing this preferred method is depictedin FIG. 8 , where a liquid CO₂ storage tank 104 may feed liquid CO₂ intothermally insulated hose 804 through gate valve 112. Both gate valve 112and flow meter 808 may control the flow rate of liquid CO₂ intothermally insulated hose 804. The thermally insulated hose 804 isconnected to expansion nozzle 816 (also referred to herein as a throttle816 or a throttle valve 816), where liquid CO₂ may be throttled and soas to induce a phase change from liquid to a mix of solid and gaseousstates. The resulting mixture of solid and gaseous CO₂ is propelledthrough exit stream 828, where Ca(OH)₂ particles are added via screwfeeder 824. Through heterogeneous nucleation, dry ice-covered Ca(OH)₂particles are created, which are then introduced into cyclone 832 wheresublimation occurs. Any remaining/excess CO₂ and CO₂ from thesublimation may be collected though gaseous CO₂ outlet 140, and returnedto liquid CO₂ storage tank 104 after going through a phase change fromgaseous state to liquid state through in-line pressurization system 152.The produced calcium carbonate coated hydroxide particles are collectedvia the connected calcium carbonate-coated calcium hydroxide productparticle outlet 144.

As previously indicated flow meter 808 and gate valve 112 control therate at which liquid CO₂ is introduced into thermally insulated hose804. The flow rate of the liquid CO₂ entering throttle 816 isproportional to the kinetic energy of exit stream 828, where a highkinetic energy of exit stream 828 may be achieved due to the initialflow rate of the liquid CO₂ and the pressure differential between theentrance of throttle 816 and the exit of throttle 816 where exit stream828 begins. A high kinetic energy of exit stream 828 allows for thesuspension of solid dry ice and also Ca(OH)₂ particles. In a preferredembodiment, the flow rate of the liquid CO₂ in thermally insulated hose804 and upon entering throttle 816 is approximately 173.5 kg/d. Thespeed of the mixture of solid and gaseous CO₂ propelled through exitstream 828 measured in proximity to the exit of throttle 816 may rangebetween 6 m/s to 600 m/s. In a preferred embodiment, the speed of themixture of solid and gaseous CO₂ in exit stream 828 measured inproximity to the exit of throttle 816 may be 60 m/s.

In the current embodiment, thermally insulated hose 804 allows for theflow of liquid CO₂ from liquid CO₂ storage tank 104 to throttle valve816. Thermally insulated hose 804 also ensures that the liquid CO₂ thatis flowing through is kept at a pressure range of 0.518 MPa to 16 MPaand a temperature range of —56.56° C. to 30.98° C. at position 812 priorto liquid CO₂ entering throttle valve 816. Furthermore, thermallyinsulated hose 804 may provide additional distance for liquid CO₂ toreach a specific flow rate. However, if the liquid CO₂ is kept at saidpressure range and temperature range within liquid CO₂ storage tank 104,and the liquid CO₂ may be discharged as a specific flow rate, insulatedhose 804 may be optional. In alternate embodiments, liquid CO₂ fromliquid CO₂ storage tank 104 may be introduced directly into throttle816, where flow meter 808 and gate valve 112 control the rate at whichliquid CO₂ is introduced into throttle 816.

Between position 812 and position 820, liquid CO₂ is throttled throughthrottle valve 816 and undergoes a phase change from liquid to a mixtureof gas and solid. More specifically, the liquid CO₂ is changed into amixture of gaseous CO₂ and solid dry ice. Undergoing a phase changeusing throttle 816 is advantageous due its simplicity. The throttlingoccurs at approximately constant enthalpy, also known as an isenthalpicprocess, per energy balance on throttle valve 816. The phase change isinduced through a change of pressure or a change of temperature, whichcan be determined through FIG. 7 . In the current embodiment, atposition 820, the pressure is below 0.518 MPa. A person skilled in theart will recognize the different configurations and variables, such astemperature and pressure, of inducing phase change from the liquid CO₂to a mixture of gaseous CO₂ and solid dry ice.

As the mixture of gaseous CO₂ and solid dry ice leave throttle valve816, the mixture enters exit stream 828. Exit stream 828 has highkinetic energy due to throttle valve 816 and also the initial kineticenergy from liquid CO₂ enter thermally insulated hose 804 from liquidCO₂ storage 104. The high kinetic energy allows the particles of Ca(OH)₂introduced from screw feeder 824 to be suspended as they flow along exitstream 828. Exit stream 828 may be encompassed by an insulated hose,pipe or any form of physical structure that will not impede the highkinetic energy of exit stream 828, while being able to maintain thetemperature and pressure as required in exit stream 828, and direct theflow of exit stream 828 towards cyclone 832. Screw feeder 824 is usedfor the introduction of Ca(OH)₂ particles to ensure a steady and regularflow of Ca(OH)₂ particles into exit stream 828. In a preferredembodiment, screw feeder 824 is in proximity to the exit of throttlevalve 816 and the beginning of exit stream 828, where kinetic energy isat its highest after exiting throttle valve 816, and also allowing timewithin exit stream 828 for heterogeneous nucleation, which will befurther discussed below. Other forms of feeder or introducing Ca(OH)₂particles may be contemplated, as long as the introduction of theCa(OH)₂ particles are done in a regular and controlled manner.

As the Ca(OH)₂ is introduced via screw feeder 824 into exit stream 828,the Ca(OH)₂ particles act as heterogeneous nucleation sites for the dryice. The dry ice forms around the Ca(OH)₂ particles and reacts with theouter shell of the Ca(OH)₂ particles producing a shell of CaCO₃. Due tothe high kinetic energy, the Ca(OH)₂ particles are suspended in the gasin exit stream 828, allowing the Ca(OH)₂ particles to act as a core andexposing the entire surface of the Ca(OH)₂ particles, allowing for auniform coating of dry ice. As the Ca(OH)₂ particles and the mixture ofgaseous CO₂ and solid dry ice travel through exit stream 828, and asheterogeneous nucleation occurs around the Ca(OH)₂ particles,sublimation may also occur, where any excess dry ice in the exit stream828 that does not undergo heterogeneous nucleation around the Ca(OH)₂particles, and any excess dry ice that has grown as a result ofheterogeneous nucleation around the Ca(OH)₂ particles may undergo aphase change into gaseous CO₂. Similarly, Ca(OH)₂ particles that undergoheterogeneous nucleation early on after entering exit stream 828 fromscrew feeder 824 may begin reacting and becoming dry ice coatedcore-shell calcium hydroxide-calcium carbonate (CSCC) particles. Inaddition, changes in temperature and pressure within exit stream 828 maycause gaseous CO₂ to become dry ice as it travels through exit stream828. Furthermore, while the high kinetic energy promotes heterogeneousnucleation of the Ca(OH)₂, there may still be a minority of Ca(OH)₂particles that remain uncoated. As such, the resulting mixtureintroduced into cyclone 832 may include gaseous CO₂, solid dry ice,Ca(OH)₂ particles, dry ice covered Ca(OH)₂ particles, and dry icecovered CSCC particles.

As said mixture enters and spirals within cyclone 832, sublimationcontinues to occur, where any excess dry ice, whether coated on theCa(OH)₂ particles or excess dry ice from the phase change from throttlevalve 816 that was introduced into cyclone 832 from exit stream 828 mayundergo a phase change into gaseous CO₂. In addition, uncoated Ca(OH)₂particles may undergo heterogeneous nucleation within cyclone 832 ifheterogeneous nucleation did not occur within exit stream 828.Furthermore, the Ca(OH)₂ particles coated in dry ice continue to reactto create dry ice covered CSCC particles. The mixture in cyclone 832 mayalso undergo an increase in temperature and a pressure drop while incyclone 832. The increase in temperature may be due to a lack ofinsulation surrounding cyclone 832, or it may be due the addition ofheating elements to increase the speed of sublimination. The pressuredrop arises due to the shape and design of cyclone 832. Similar tomethod 200, the residence time of the dry ice coated CSCC particleswhile undergoing sublimation will affect the thickness of the calciumcarbonate coating on the calcium hydroxide particles. The residence timeof the dry ice coated CSCC particles in cyclone 832 may be affected byvarious factors, including the shape and design of cyclone 832.

Cyclone 832 further separates gaseous CO₂ from the CaCOs-coated Ca(OH)₂particles through cyclonic separation and/or centrifugal force, wheredue to the weight of the particles or as a result of the CaCO₃ particleslosing momentum when colliding against the wall of cyclone 832, theCaCO₃-coated Ca(OH)₂ particles settle at the bottom of cyclone 832 dueto gravity and are collected at the bottom of cyclone 832, at calciumcarbonate-coated calcium hydroxide product particle outlet 144. In thecurrent embodiment, collector 836 collects the CaCOs-coated Ca(OH)₂particle product, however as will be evident, collector 836 is optional,and if present, may be of any shape or size for the collection of theCaCOs-coated Ca(OH)₂ particle product.

Gaseous CO₂ is collected at the top of cyclone 832 at gaseous CO₂ outlet140, due to the spinning effect of cyclone 832. Similar to embodiment ofsystem 100C, the gaseous CO₂ that is collected at gaseous CO₂ outlet 140may be returned to storage tank 104 via return line 156, after goingthrough in-line pressurization system 152. As previously discussed,in-line pressurization system 152 raises the pressure within return line156 to induce a phase change on the CO₂, converting the gaseous CO₂ intoliquid CO₂.

In alternative embodiments, cyclone 832 may be replaced withelectrostatic precipitators or separator vessel 136 to allow forsublimination of dry ice and the separation of dry ice covered CSCCparticles from the remaining mixture. A person skilled in the art willrecognize that different equipment may be used to allow for sublimationof dry ice and the separation of dry ice covered CSCC particles fromgaseous CO₂ and other mixture components.

Referring to FIG. 9 , method 900 is depicted for producing calciumcarbonate coated calcium hydroxide particles using system 800. At block905, liquid CO₂ is discharged from liquid CO₂ storage tank 104 throughgate valve 112, thermally insulated hose 804, and throttle valve 816into exit stream 828, where the liquid CO₂ undergoes a phase change asit is throttled through throttle valve 816 from liquid CO₂ into amixture of gaseous CO₂ and solid dry ice.

As the mixture of gaseous CO₂ and solid dry ice travels through exitstream 828, screw feeder 824 introduces Ca(OH)₂ particles at a regularand controlled rate into exit stream 828, where it joins the mixture ofgaseous CO₂ and solid dry ice. This is depicted at block 910.

At block 915, as the Ca(OH)₂ particles travel through exit stream 828,the Ca(OH)₂ particles act as heterogeneous nucleation sites for dry ice.As previously discussed, due to the high kinetic energy of exit stream828, the Ca(OH)₂ particles are suspended in the air, allowing theexposure of the surface of the Ca(OH)₂ particles for the build-up andformation of dry ice around the Ca(OH)₂ particles. Once covered with dryice, particle agglomeration of the dry ice covered Ca(OH)₂ particles islimited.

At block 920, the dry ice covered Ca(OH)₂ particles are introduced intocyclone 832 for cyclonic separation. While being exposed to therotational effects within cyclone 832, the dry ice covered Ca(OH)₂particles and the excess solid dry ice from exit stream 828 that did notform around the Ca(OH)₂ particles are sublimated, changing the phase ofthe dry ice into gaseous CO₂. This is depicted at block 925.

Through cyclonic separation, the gaseous CO₂ and the calcium carbonatecoated calcium hydroxide particles are separated, with the gaseous CO₂discharged through the top of cyclone 832 through gaseous CO₂ outlet 140(as depicted at block 935), and the calcium carbonate coated calciumhydroxide particles falling to the bottom of cyclone 832 and collectedthrough calcium carbonate coated calcium hydroxide particle outlet 144(as depicted at block 930).

In certain embodiments, the collected gaseous CO₂ may be optionallyrecycled by inducing a phase change from gaseous CO₂ to liquid CO₂ asdepicted at block 940, where the liquid CO₂ may be returned to liquidCO₂ storage tank 104 to be discharged again at block 905.

Although the foregoing description and accompanying drawings relate tospecific preferred embodiments of the present invention as presentlycontemplated by the inventor, it will be understood that variouschanges, modifications and adaptations, may be made without departingfrom the spirit of the invention.

1. A method of preparing calcium carbonate (CaCO₃)-coated calciumhydroxide (Ca(OH)₂) particles comprising: introducing liquid carbondioxide into a reaction vessel; introducing calcium hydroxide particlesinto the reaction vessel; effectively mixing the calcium hydroxideparticles into the liquid carbon dioxide; inducing a phase change in theliquid carbon dioxide so as to coat the calcium hydroxide particles indry ice; and sublimating the dry ice after a predetermined residencetime to control the thickness of the calcium carbonate coating on thecalcium hydroxide particles.
 2. The method of claim 1, wherein theliquid carbon dioxide introduced into the reaction vessel is at apressure of 8 MPa and a temperature of -25° C.
 3. The method of claim 1,wherein the liquid carbon dioxide is introduced into the reaction vesselat a pressure range of 0.518 MPa to 16 MPa and a temperature range of-56.56° C. to 30.98° C.
 4. The method of claim 1, wherein introducingcalcium hydroxide particles into the reaction vessel includes feedingthe calcium hydroxide particles into an auxiliary chamber, flushing thecalcium hydroxide particles in the auxiliary chamber with the liquidcarbon dioxide and introducing the mixture into the reaction vessel tobe further mixed with the already present liquid carbon dioxide.
 5. Themethod of claim 1, wherein the calcium hydroxide particles areintroduced into the reaction vessel prior to the liquid carbon dioxidebeing introduced into the reaction vessel.
 6. The method of claim 1,wherein the reaction vessel is a high-pressure reactor, thehigh-pressure reactor further comprising a stirrer for mixing.
 7. Themethod of claim 1, wherein the reaction vessel is an inline mixer. 8.The method of claim 1, wherein inducing the phase change in the liquidcarbon dioxide is performed using a throttle valve to flash the liquidcarbon dioxide into dry ice.
 9. The method of claim 8 wherein thethrottle valve flashes at an exit pressure of 0.1 MPa.
 10. The method ofclaim 8, wherein the throttle valve flashes at an exit pressure range of0.01 MPa to 0.518 MPa and a temperature lower than -56.56° C.
 11. Themethod of claim 1, wherein controlling the thickness of the calciumcarbonate coating on the calcium hydroxide particles occurs over thepredetermined residence time in a separator vessel at a pressure of lessthan or equal to 0.518 MPa.
 12. The method of claim 1, furthercomprising: collecting gaseous carbon dioxide from the sublimation ofthe dry ice; and inducing a phase change in the gaseous carbon dioxideto provide liquid carbon dioxide to be introduced into the reactionvessel.
 13. A system for producing calcium carbonate (CaCO₃)-coatedcalcium hydroxide (Ca(OH)₂) particles comprising: a reaction vessel forreceiving liquid carbon dioxide and calcium hydroxide particles; astirrer to effectively mix the liquid carbon dioxide and calciumhydroxide particles; a throttle valve for inducing a phase change toliquid carbon dioxide to coat the calcium hydroxide particles in dryice; and a separator vessel for sublimating the dry ice after apredetermined residence time to control the thickness of the calciumcarbonate coating on the calcium hydroxide particles.
 14. The system ofclaim 13, wherein the liquid carbon dioxide received by the reactionvessel is at a pressure of 8 MPa and a temperature of -25° C.
 15. Thesystem of claim 13, wherein the liquid carbon dioxide is received at thereaction vessel at a pressure range of 0.518 MPa to 16 MPa and atemperature range of -56.56° C. to 30.98° C.
 16. The system of claim 13,wherein the calcium hydroxide particles are received by the reactionvessel through flushing the calcium hydroxide particles in an auxiliarychamber with the liquid carbon dioxide and introducing the mixture intothe reaction vessel to be mixed with the already present liquid carbondioxide.
 17. The system of claim 13, wherein the calcium hydroxideparticles are received by the reaction vessel prior to the liquid carbondioxide being received by the reaction vessel.
 18. The system of claim13, wherein the liquid carbon dioxide is received by the reaction vesselprior to the calcium hydroxide particles are received by the reactionvessel.
 19. The system of claim 13, wherein the reaction vessel is ahigh-pressure reactor, the high-pressure reactor further comprising astirrer for mixing.
 20. The system of claim 13, wherein the vessel is aninline mixer.
 21. The system of claim 13, wherein the throttle valveinduces a phase change by flashing the liquid carbon dioxide at 0.1 MPa.22. The system of claim 13, wherein the throttle valve induces a phasechange by flashing the liquid carbon dioxide at a pressure range of 0.1MPa to 0.518 MPa and a temperature lower than -56.56° C.
 23. The systemof claim 13, wherein controlling the thickness of the calcium carbonatecoating on the calcium hydroxide particles occurs over the predeterminedresidence time in the separator vessel at a pressure of less than orequal to 0.518 MPa.
 24. The system of claim 13, the system furthercomprising: a gaseous carbon dioxide outlet connected to the separatorvessel, the gaseous carbon dioxide outlet to collect gaseous carbondioxide from the sublimation of the dry ice in the separator vessel; areturn line with an in-line pressurization system connecting the gaseouscarbon dioxide outlet and the reaction vessel, the return line with thein-line pressurization system configured to induce a phase change to thegaseous carbon dioxide to provide liquid carbon dioxide to be introducedinto the reaction vessel.
 25. A method of preparing calcium carbonate(CaCO₃)-coated calcium hydroxide (Ca(OH)₂) particles comprising:throttling a liquid carbon dioxide into an exit stream so as to induce aphase change in the liquid carbon dioxide to a mixture of a gaseouscarbon dioxide and a solid dry ice; introducing calcium hydroxideparticles to the exit stream so as to induce heterogeneous nucleation ofthe calcium hydroxide particles and promote the formation of the dry iceparticles around the calcium hydroxide particles; and sublimating thedry ice particles formed around the calcium hydroxide particles.
 26. Asystem for producing calcium carbonate (CaCO₃)-coated calcium hydroxide(Ca(OH)₂) particles comprising: a throttle valve for inducing a phasechange from a liquid carbon dioxide to a mixture of a gaseous carbondioxide and a solid dry ice, the throttle valve being operable todischarge the mixture into an insulated hose comprising an exit stream;a screw feeder for introducing calcium hydroxide particles to the exitstream, the calcium hydroxide particles being the heterogenoeusnucleation agent for inducing heterogeneous nucleation and promoting dryice formation around the calcium hydroxide particles; and a cyclone toreceive the mixture from the exit stream and the dry ice covered calciumhydroxide particles, the cyclone to further sublimate the dry ice.